Microsporidia are obligate intracellular protozoan parasites that are commonly found to infect insects and fish, and more recently, are seen to cause opportunistic infections in persons with AIDS. Little is known about the prevalence and natural history of microsporidiosis. The purpose of these studies is to determine the prevalence of microsporidiosis in selected populations, and to retrospectively, evaluate the natural history of the infected individuals. In one study, stool specimens were examined histochemically (Calcofluor White stain, and Modified Trichrome Blue stain) for the presence of microsporidia in stool specimens of HIV infected individuals submitted to Charity Hospital, New Orleans, LA during the last four months of 1996. Six of 136 specimens (4.4%) were positive for microsporidia (4 for Enterocytozoon bieneusi, and 2 for Encephalitozoon intestinalis, based on size presumption). Case histories and longitudinal follow up studies are continuing. In a second study, 207 serum and stool samples from the private practice of a gastroenterologist in the Los Angeles, CA area were assayed. By histochemical staining of the stool specimens, 27 (13%) were found to be positive, while the serum ELISA results were variable, and did not correlate with results of the histochemistry diagnostics. Anecdotal case report studies indicated that Encephalitozoon hellem, which was only found to infect persons with AIDS, has also been found in parakeets. Encephalitozoon cuniculi strain III, which has only been isolated and cultured from dogs, can now also be found in persons with AIDS. These results suggest that microsporidiosis may be more common than first believed, and that in some circumstances, microsporidiosis may be zoonotic. PCR based methods, which are believed to be more sensitive than histochemical methods, will later be employed to determine if additional cases of microsporidiosis are detected. In addition, we are participating as a central lab for diagnosing microsporidiosis in a clinical trial (ACTG 336; Study of nitazoxanide for persons with AIDS and cryptosporidiosis). Plans are in progress to retrospectively assess the natural history of persons with microsporidiosis.